Lubricant composition for automobile driving system

ABSTRACT

A lubricant composition having a 40° C. dynamic viscosity of 40 mm 2 /s or less that has wear resistance equal to or better than a lubricant having a 40° C. dynamic viscosity of 76 mm 2 /s is achieved by providing a base oil with a zinc dithiophosphate and alkaline earth metal salt in an amount to provide a ratio of elemental zinc to alkaline earth metal in the oil in the range of 0.2 to 1.0.

This application claims the benefit of Japanese Patent Application 2004-163106 filed Jun. 1, 2004 (Patent Applicants: Tonen General Sekiyu K.K. and Toyota Motor Corporation).

FIELD OF THE INVENTION

The present invention pertains to a lubricant composition for an automobile driving system. More specifically, the present invention pertains to a lubricant composition for automobile gears, especially for a manual speed-change gear.

BACKGROUND OF THE INVENTION

In recent years, as a measure for preventing global warming, various schemes for protecting the environment have been proposed. One said scheme calls for development of an environmentally friendly lubricant. An environmentally friendly lubricant for use in automobiles, is required to have an excellent effect in improving gas mileage to reduce the amount of carbon dioxide exhaust from internal combustion engines. In order to increase the mileage with the lubricant, two methods have been under study, that is, a method for reducing friction in the sliding parts and a method for reducing the viscosity of the lubricant.

Concerning reducing friction, it has been proposed that, in order to increase the effect of the gear lubricant used in the power transmission system for increasing the mileage, a gear lubricant composition prepared using molybdenum dithiophosphate or another friction-reducing agent is used (see Japanese Kokoku Patent Application No. Hei 6[1994]-33390). In another method, a combination of a prescribed polymethacrylate-based viscosity index increasing agent and a molybdenum-based friction-reducing agent is used to obtain a lubricant composition that can maintain a low friction coefficient even after oxidation degradation (see Japanese Patent No. 2906024).

However, for the manual speed-change gear of an automobile, friction between metal parts is exploited to form a synchronization device incorporated within it. In order to realize smooth operation of the synchronization device, lowering of the friction coefficient is undesirable because if only the viscosity is reduced, the oil film becomes thinner, and wear is facilitated. Aluminum parts adopted for reducing the weight of the driving system device are especially more prone to wear than are steel parts, and they are more easily affected by a reduced viscosity. In practice, almost all commercially available lubricants for manual speed-change gear of automobiles have a dynamic viscosity at 40° C. (hereinafter referred to as “40° C. dynamic viscosity”) higher than 40 mm²/s, and a lower-viscosity manual speed-change gear lubricant has not been used in practical application.

On such background, there is a high demand for development of a lubricant composition for a manual speed-change gear that exploits technology for increasing mileage by lowering the viscosity of the lubricant.

DISCLOSURE OF THE INVENTION

The objective of the present invention is to solve the aforementioned problems relating to the prior art for improving mileage by providing a type of lubricant composition for automobile gears, especially a lubricant composition for a manual speed-change gear, characterized by the fact that the mileage can be improved by lowering the viscosity of the lubricant and, at the same time, the durability of wear resistance can be maintained.

SUMMARY OF THE INVENTION

In order to solve the aforementioned problems, the present inventors have performed extensive research. As a result of this research, it was found that when an alkaline earth metal salt and zinc dithiophosphate are mixed at a prescribed ratio, it is possible to obtain a lubricant composition that has a 40° C. dynamic viscosity of 40 mm²/s or lower, and, at the same time, that has excellent wear resistance for steel parts and, especially, for aluminum parts, equal to or better than the wear resistance of a conventional commercially available lubricant with a 40° C. dynamic viscosity of 76 mm²/s. The present invention was achieved based on this finding.

That is, the present invention provides a lubricant composition for a manual speed-change gear characterized by the following facts: the lubricant composition for a manual speed-change gear contains a base oil, and the following components added into said base oil: (a) at least one alkaline earth metal salt selected from the group of alkaline earth metal salts of sulfonate, salicylate, and phenolate, and (b) zinc dithiophosphate;

-   the dynamic viscosity at 40° C. of said lubricant composition is 40     mm²/s or lower; -   the content of said alkaline earth metal salt relative to the total     weight of said lubricant composition corresponds to a content of the     alkaline earth metal element in the oil of 0.1 wt % or more; -   and the ratio of the quantity of elemental zinc in the oil to the     quantity of elemental alkaline earth metal in the oil is 0.2 to 1;     and -   where the quantity of elemental zinc in the oil is derived from said     zinc dithiophosphate, and the quantity of alkaline earth metal     element in the oil is derived from said organic acid alkaline earth     metal salt.

DETAILED DESCRIPTION OF INVENTION

As explained above, according to the present invention, by adding an organic acid alkaline earth metal salt, especially magnesium sulfonate, in a prescribed quantity in a base oil with a low viscosity, and by mixing said organic acid alkaline earth metal salt and zinc dithiophosphate at a prescribed ratio, it is possible to obtain a type of lubricant for a manual speed-change gear that can display significant wear resistance for not only steel parts but also aluminum sliding parts, and that has an excellent effect in increasing mileage.

As explained above, the present invention provides a lubricant composition for automobile gears, especially a lubricant composition for a manual speed-change gear, characterized by the fact that it is composed of a low-viscosity base oil as well as an organic acid alkaline earth metal salt and zinc dithiophosphate in a prescribed ratio. The preferable embodiments are the following (1)-(6):

-   (1) The aforementioned lubricant composition for a manual     speed-change gear preferably is characterized by the fact that the     dynamic viscosity at 40° C. of said lubricant composition is 30     mm²/s or lower. -   (2) The aforementioned lubricant composition for a manual     speed-change gear preferably is characterized by the fact that said     alkaline earth metal salt is an alkaline earth metal salt of     sulfonate. -   (3) The aforementioned lubricant composition for a manual     speed-change gear preferably is characterized by the fact that said     alkaline earth metal salt of sulfonate is magnesium sulfonate. -   (4) The aforementioned lubricant composition for a manual     speed-change gear preferably is characterized by the fact that the     total base value of said alkaline earth metal salt is 200 mgKOH/g or     higher. -   (5) A lubricant composition for a manual speed-change gear     preferably is characterized by the following facts:

it contains a base oil as well as magnesium sulfonate and zinc dithiophosphate added into said base oil;

the dynamic viscosity at 40° C. of the lubricant composition is 30 mm²/s or lower;

the content of said organic acid alkaline earth metal salt relative to the total weight of said lubricant composition corresponds to a content of the alkaline earth metal element in the oil of 0.1 wt % or more;

and the ratio of the quantity of element zinc in the oil to the quantity of alkaline earth metal element in the oil is form 0.3 to 0.8 and where, the quantity of elemental zinc in the oil is derived from said zinc dithiophosphate, and the quantity of alkaline earth metal element in the oil is derived from said organic acid alkaline earth metal salt.

-   (6) A lubricant composition for a manual speed-change gear     preferably is characterized by the following facts: it contains a     base oil as well as the following components added into said base     oil:

(a) at least one acid alkaline earth metal salt selected from the group of alkaline earth metal salts of sulfonate, salicylate, and phenolate, and

(b) zinc dithiophosphate;

the dynamic viscosity at 40° C. of the lubricant composition is 40 mm²/s or lower;

the content of said organic acid alkaline earth metal salt relative to the total weight of said lubricant composition corresponds to a content of the alkaline earth metal element in the oil of 0.1 wt % or more;

and the ratio of the quantity of element zinc in the oil to the quantity of alkaline earth metal element in the oil is from 0.2 to 1.0 and where, the quantity of elemental zinc in the oil is derived from said zinc dithiophosphate, and the quantity of alkaline earth metal element in the oil is derived from said organic acid alkaline earth metal salt.

In the following, explanation will be provided for the structural components of the lubricant composition for a manual speed-change gear in the present invention. The lubricant composition for a manual speed-change gear contains a base oil as well as an organic acid alkaline earth metal salt, zinc dithiophosphate, and other additives for a speed-change gear that maintain the extreme-pressure performance, etc., added to the base oil.

The base oil as a structural component of the lubricant composition for a manual speed-change gear may be a conventional base oil for a lubricant or another usable type, and there is no special limitation on the type. More specifically, examples include mineral oil base oils, GTL (gas to liquid)-based base oil, synthetic oil-based base oil, as well as mixed base oils.

Examples of mineral oil base oils include solvent refined mineral oils or hydrogenation treated oils and other mineral oils prepared by treatment of a lubricant distillation fraction obtained by reduced pressure distillation of residual oil from an ambient pressure distillation device for paraffin-based, intermediate-based, or naphthene-based feed oil by means of solvent refinement, hydrogenation decomposition, hydrogenating treatment, hydrogenating refinement, solvent dewaxing, contact dewaxing, white clay treatment or another refinement method, mineral oil prepared by treatment in said refinement process of de-bitumen oil prepared by solvent de-bitumen treatment of reduced pressure distillation residual oil, mineral oil obtained by isomerizing a wax component, as well as mixed oils thereof. In said solvent refinement, phenol, furfural, N-methyl-2-pyrrolidone, or another aromatic extracting solvent is used. Also, examples of solvents for solvent dewaxing include liquefied propane, MEK (methyl ethyl ketone)/toluene, and the like. On the other hand, in contact dewaxing, for example, a shape-selecting zeolite or the like may be used as the dewaxing solvent.

Examples of refined base oil substrates prepared in the above include different types of light neutral oils, middle neutral oils, heavy neutral oils, bright stock, etc., having different viscosity levels. One may blend said substrates appropriately to prepare the mineral oil-type base oil.

Examples of GTL-type base oils include the lubricant fraction separated from liquid product obtained from natural gas or another raw material using a GTL process, the lubricant fraction obtained by means of hydrogenation decomposition of generated wax, etc. In addition, one may also use the lubricant fraction separated from liquid oil generated in an ATL (asphalt to liquid) process using asphalt or another heavy residual oil component as the raw material.

On the other hand, as a synthetic oil base oil, one may select from the following group of compounds to obtain an appropriate viscosity property for the lubricant composition for a manual speed-change gear: a poly(α-olefin) (such as poly(1-hexene), poly(1-octene), poly(1-decene), and their mixtures); polybutene; an ethylene-alkylene copolymer; an alkyl benzene (such as dodecylbenzene, tetradecylbenzene, di(2-ethylhexyl)benzene, dinonylbenzene, etc.); a polyphenyl (such as biphenyl, alkylated polyphenyl, etc.); an alkylated diphenylether, an alkylated diphenyl sulfide, and their derivatives; esters formed from dibasic acids (such as phthalic acid, succinic acid, alkyl succinic acid, alkenyl succinic acid, maleic acid, azelaic acid, suberic acid, sebatic acid, fumaric acid, adipic acid, linoleic acid dimer, etc.) and various alcohols (such as butyl alcohol, hexyl alcohol, 2-ethylhexyl alcohol, dodecyl alcohol, ethylene glycol, diethylene glycol monoether, propylene glycol, etc.); esters formed from C5-18 monocarboxylic acids and polyols (such as neopentyl glycol, trimethylol propane, pentaerythritol, dipentaerythritol, tripentaerythritol, and the like); as well as a polyoxyalkylene glycol, a polyoxyalkylene glycol ester, a polyoxyalkylene glycol ether, a phosphate, etc.

As explained above, the base oil of the lubricant composition for a manual speed-change gear in the present invention is prepared by selecting one of said various types of base oil substrates, either alone or as a mixture of several, such that the 40° C. dynamic viscosity of the lubricant composition is 40 mm²/s or lower, or preferably 30 mm²/s or lower. The base oil has the desired viscosity and other properties required for a lubricant. Consequently, the viscosity of the base oil should be appropriate to provide a lubricant composition of the present invention. The viscosity depends on the composition of the additives, etc., and can be selected preferably from those having a 40° C. dynamic viscosity in the range of 25-40 mm²/s.

The organic acid alkaline earth metal salt is selected from the group of alkaline earth metal salts of sulfonate, phenolate and salicylate.

The alkaline earth metal sulfonate is an alkaline earth metal salt of a petroleum sulfonic acid, long-chain alkylbenzene sulfonic acid, and alkyl napththalene sulfonic acid. It is a component of the composition for a manual speed-change gear of the present invention. A typical example is represented by formula (1):

In the formula, M represents an alkaline earth metal, such as magnesium, calcium, or barium. Among them, magnesium is especially preferred. R¹ and R² are C1-30 hydrocarbon groups, which may be identical or different from each other. At least one of the hydrocarbon groups should be a C6 or higher alkyl group. Examples of preferable hydrocarbon groups include C1-18 straight chain or branched alkyl groups; C2-18 straight chain or branched alkenyl groups; C6-30 cycloalkyl groups; C6-18 aryl groups, etc. The aryl groups are optionally substituted with C1-12 alkyl groups or C2-12 alkenyl groups. Especially preferable hydrocarbon groups include C6-18 straight-chain or branched alkyl groups.

For the sulfonate in the lubricant composition for a manual speed-change gear of the present invention, perbasic salts are preferred. However, it is also possible to use a normal salt or basic salt. A perbasic salt has excess hydroxide or carbonate dispersed in colloidal form in the sulfonate. It is preferred that the total base value be 200 mgKOH/g or higher.

The quantity of alkaline earth metal sulfonate should be appropriate so that the alkaline earth metal quantity in the oil with respect to the total weight of the composition is 0.1 wt % or more, or preferably in the range of 0.15-0.6 wt %, or more preferably in the range of 0.15-0.3 wt %.

As the alkaline earth metal sulfonate, magnesium sulfonate is especially preferred. In the low-viscosity state, when it is used together with zinc dithiophosphate, excellent wear resistance can be displayed in a manual speed-change gear having sliding aluminum parts.

The alkaline earth metal phenolate includes an alkaline earth metal salt of alkyl phenol sulfide represented by formula (2):

In the formula, R¹, R², R³ and R⁴ represent C1-30 hydrocarbon groups, which may be identical or different from each other. At least one of the hydrocarbon groups should be a C6 or higher alkyl group. Examples of preferable hydrocarbon groups include C1-18 straight-chain or branched alkyl groups; C2-18 straight-chain or branched alkenyl groups; C6-30 cycloalkyl groups; C6-18 aryl groups, etc. The aryl groups are optionally substituted with C1-12 alkyl groups or C2-12 alkenyl groups. Especially preferred hydrocarbon groups include C6-18 straight-chain or branched alkyl groups. x is an integer in the range of 1-3. M represents magnesium, calcium, or barium. Among them, magnesium is especially preferred.

A perbasic salt of phenolate is obtained by dispersing a hydroxide or carbonate in colloidal form in phenolate, just as with sulfonate. The total base value is preferably 200 mgKOH/g or larger. However, one may also use a normal salt or basic salt.

The quantity of alkaline earth metal phenolate with respect to the base oil should be appropriate to correspond to a quantity of alkaline earth metal element in the oil with respect to the total weight of the composition of 0.1 wt % or more, or preferably in the range of 0.15-0.6 wt %.

The alkaline earth metal salicylate is represented by a compound represented by formula (3):

where, M represents an alkaline earth metal, such as magnesium, calcium, or barium. Among them, magnesium is especially preferred. R¹ and R² represent C1-30 hydrocarbon groups, which may be identical or different from each other. At least one of the hydrocarbon groups should be a C6 or higher alkyl group. Examples of preferable hydrocarbon groups include C1-8 straight-chain or branched alkyl groups; C2-18 straight-chain or branched alkenyl groups; C6-30 cycloalkyl groups; C6-18 aryl groups, etc. The aryl groups are optionally substituted with C1-12 alkyl groups or C2-12 alkenyl groups. Especially preferable hydrocarbon groups include C6-18 straight-chain or branched alkyl groups.

For the lubricant composition for a manual speed-change gear of the present invention, perbasic salicylate is preferred. However, one may also use normal salt or basic salt. A perbasic salt is prepared by dispersing carbonate in colloidal form in salicylate.

The quantity of alkaline earth metal salicylate with respect to the base oil should be appropriate to correspond to a quantity of the alkaline earth metal in the oil with respect to the total weight of the composition of 0.1 wt % or more, or preferably in the range of 0.15-0.6 wt %.

As explained above, the structural components of the lubricant composition for a manual speed-change gear of the present invention include said alkaline earth metal sulfonate, phenolate and salicylate. Among them, especially, the alkaline earth metal salt of sulfonate is preferred. More specifically, magnesium sulfonate is preferred.

In the following, explanation will be provided for zinc dithiophosphate as a structural component of the lubricant composition for a manual speed-change gear of the present invention.

An example of zinc dithiophosphate is a compound represented by following formula (4):

In formula (4), R₁ and R₂ represent C1-20 hydrocarbon groups, which may be identical or different from each other. Examples of hydrocarbon groups include C1-20 alkyl groups; C2-20 alkenyl groups; C6-20 cyclohexyl groups, aryl groups, alkyl aryl groups, aryl alkyl groups, etc. Specific examples include a methyl group, ethyl group, propyl group, butyl group, pentyl group, hexyl group, heptyl group, octyl group, 2-ethylhexyl group, nonyl group, decyl group, undecyl group, dodecyl group, tridecyl group, tetradecyl group, pentadecyl group, hexadecyl group, heptadecyl group, octadecyl group, stearyl group, oleyl group, butylphenyl group, nonylphenyl group, etc., as well as branched alkyl groups thereof, etc. Preferable hydrocarbon groups are C3-18 alkyl groups. Examples of alkyl groups include primary and secondary alkyl groups. More specifically, it is preferred that compounds having the following groups be used: isopropyl group, isobutyl group, secondary butyl group, pentyl group, hexyl group, 4-methyl-2-pentyl group, octyl group, 2-ethylhexyl group, nonyl group, decyl group, as well as dodecyl group, tridecyl group, tetradecyl group, hexadecyl group, octadecyl group, and other alkyl groups.

Typical examples of zinc dithiophosphate include zinc diisopropyl dithiophosphate, zinc diisobutyl dithiophosphate, zinc di-secondary butyl thiophosphate, zinc di(n-pentyl) dithiophosphate, zinc di(n-hexyl) dithiophosphate, zinc di(4-methyl-2-pentyl) dithiophosphate, zinc di(n-octyl) dithiophosphate, zinc di(2-ethylhexyl) dithiophosphate, zinc di(n-nonyl) dithiophosphate, zinc di(n-decyl) dithiophosphate, zinc di(n-dodecyl) dithiophosphate, zinc di(n-tridecyl) dithiophosphate, zinc di(n-tetradecyl) dithiophosphate, zinc di(n-hexadecyl) dithiophosphate, zinc di(n-octadecyl) dithiophosphate, etc. According to the present invention, for the lubricant composition for a manual speed-change gear, zinc dithiophosphate containing primary and secondary alkyl groups is preferred. For example, one may blend a zinc dithiophosphate having primary alkyl groups as the main component and a zinc dithiophosphate having secondary alkyl groups as the main component appropriately to adjust the proportions of the primary and secondary alkyl groups.

The quantity of said zinc dithiophosphate in the lubricant composition should be appropriate corresponding to an elemental zinc quantity in the oil in the range of 0.05-0.5 wt %, or preferably in the range of 0.1-0.2 wt %.

For the lubricant composition for a manual speed-change gear of the present invention, the quantity of said organic acid alkaline earth metal salt should be appropriate corresponding to an alkaline earth metal element quantity in the oil of 0.1 wt % or more; and the quantity of said zinc dithiophosphate should be appropriate corresponding to an elemental zinc quantity in the oil in the range of 0.05-0.5 wt %. Also, the ratio of the quantity of element zinc in the oil to the quantity of alkaline earth metal element in the oil is from 0.21 and where, the quantity of elemental zinc in the oil is derived from said zinc dithiophosphate, and the quantity of alkaline earth metal element in the oil is derived from said organic acid alkaline earth metal salt.

Especially, the preferred ratio of the quantity of elemental zinc in the oil to the quantity of alkaline earth metal element in the oil is from 0.3 to 0.8.

For the lubricant composition for a manual speed-change gear of the present invention, if the ratio of the quantity of elemental zinc in the oil to the quantity of alkaline earth metal element in the oil is over 1, the wear resistance decreases. On the other hand, if said ratio is less than 0.2, the wear resistance is worsened, and this is undesired.

In the following, explanation will be provided for other additives in the composition as needed, in addition to the aforementioned necessary additives.

An extreme-pressure agent is added in the lubricant composition for a manual speed-change gear of the present invention to maintain the extreme-pressure performance. In addition, as needed, one may add other additives appropriately, such as an ash-free dispersing agent, friction-adjusting agent, dissolving agent, rubber-expansion agent, fluid point lowering agent, and oxidation inhibitor. Also, other additives may be added as needed.

Examples of extreme pressure agents that may be added include an olefin polysulfide, sulfurized oils and fats, dialkyl polysulfide, and other sulfur-based compounds; alkyl and allyl phosphate, alkyl and allyl phosphite, amine phosphate, and other phosphorus-based compounds; paraffin chloride, and other chlorine-based compounds. They may be used either alone or as a mixture of several. Also, a combination of a sulfur based composition and a phosphorus based composition may be used. For example, a combination of an olefin sulfide and an alkyl phosphate may be used. The quantity is usually in the range of 0.05-3 wt %.

Examples of ash-free dispersing agents that may be used include polybutenyl succinic acid imide-based compounds, polybutenyl succinic acid amide-based compounds, benzyl amine-based compounds, succinic ester-based compounds, succinic ester-acid-based compounds, etc., usually added in a quantity in the range of 0.05-7 wt %.

Examples of friction-adjusting agents include organic molybdenum-based compounds, fatty acids, higher alcohols, fatty acid esters, oils and fats, amines, polyamide, sulfide ester, phosphates, acidic phosphates, phosphites, phosphate amine salts, etc. They are usually added in a quantity of 0.05-5 wt %.

Examples of defoaming agents that may be added include a dimethyl polysiloxane, polyacrylate, etc. They may be added appropriately in a small quantity.

Examples of fluid point decreasing agents that may be added include an ethylene-vinyl acetate copolymer, condensate of chlorinated paraffin and naphthalene, condensate of chlorinated paraffin and phenol, polymethacrylate, polyalkyl styrene, etc. Usually, the quantity is in the range of 0.1-10 wt %.

Examples of oxidation inhibitors that may be used include an alkylated diphenylamine, phenyl-α-naphthylamine, alkylated-α-naphthylamine, and other amine-based oxidation inhibitors, 2,6-ditertiary butylphenol, 4,4′-methylene bis(2,6-ditertiary butylphenol), and other phenolic oxidation inhibitors, as well as zinc dithiophosphate, etc. The quantity is usually in the range of 0.05-5 wt %.

EXAMPLES

In the following, explanation will be provided more specifically for examples of the present invention and comparative examples. However, the present invention is not limited to the examples.

A quantitative evaluation was performed using the following measurement methods. Also, the types of base oils and additives used in the examples are listed below.

METHOD FOR EVALUATION OF WEAR RESISTANCE

For each oil sample, the friction width formed on a block of the following listed material and under the following conditions was measured using a LFW-1 tester (ASTM D2714).

-   -   Test ring: S-10 (FALEX Test Ring H60)     -   Block: Aluminum sliding member     -   Test conditions:         -   Load: 5N         -   Velocity: 2 m/s         -   Temperature: 100° C.         -   Time: 1 hour             Base Oil

Refined mineral oil: 40° C. dynamic viscosity of 25-26 mm²/s

Additives

Zinc dithiophosphate (ZnDTP): Mixture having primary/secondary alkyl groups.

Magnesium sulfonate: Perbasic salt with total base value of 400 mgKOH/g.

Extreme-pressure agent, etc.: Sulfur-phosphorus-based package containing sulfur-based and phosphorus-based extreme-pressure agents, as well as an ash-free dispersing agent, friction-adjusting agent, defoaming agent, etc.

Example 1

With said refined mineral oil as the base oil, magnesium sulfonate was added at a quantity corresponding to a content of elemental Mg in the oil of 0.15 wt %, and zinc dithiophosphate was added in a quantity corresponding to a content of elemental Zn in the oil of 0.1 wt %, with the ratio of the quantity of elemental Zn in the oil to the quantity of elemental Mg in the oil adjusted to 0.67. In addition, as other additives, the sulfur-phosphorus based (S—P) package was added in a quantity of 7.1 wt %, forming oil sample A with a 40° C. dynamic viscosity of 30 mm²/s.

For oil sample A, the friction width measured using the aforementioned wear-resistance evaluation method was found to be 0.74 mm.

Example 2

Oil sample B with a 40° C. dynamic viscosity of 30 mm²/s was prepared in the same way as in Example 1, except that the magnesium sulfonate was added in a quantity corresponding to a quantity of elemental Mg in the oil of 0.3 wt %, and the zinc dithiophosphate was added in a quantity corresponding to a quantity of elemental Zn in the oil of 0.2 wt %, with the ratio of elemental Zn to elemental Mg in the oil being 0.67.

For oil sample B, the friction width measured using the aforementioned wear-resistance evaluation method was found to be 0.80 mm.

Example 3

Oil sample C with a 40° C. dynamic viscosity of 30 mm²/s was prepared in the same way as in Application Example 1, except that the magnesium sulfonate was added in a quantity corresponding to a quantity of elemental Mg in the oil of 0.3 wt %, and the zinc dithiophosphate was added in a quantity corresponding to a quantity of elemental Zn in the oil of 0.1 wt %, with the ratio of elemental Zn to elemental Mg in the oil being 0.33.

For oil sample C, the friction width measured using the aforementioned wear-resistance evaluation method was found to be 0.80 mm.

Comparative Example 1-1

A commercially available oil for a manual speed-change gear with a 40° C. dynamic viscosity of 76 mm²/s (with the ratio of elemental Zn to elemental Ca in the oil being 5.00) was used in said wear-resistance evaluation test, and the results indicated a friction width of 0.83 mm.

Comparative Example 1-2

A low-viscosity refined mineral oil was used to prepare oil sample (a) with a 40° C. dynamic viscosity of 30 mm²/s. The ratio of the quantity of elemental Zn in the oil to the quantity of elemental Ca in the oil was the same as that of the commercially available oil used in Comparative Example 1-1, that is, 5.00. The friction width of oil sample (a) measured using the wear-resistance evaluation method was found to be 1.05 mm.

Comparative Examples 2-1 through 2-6

With said refined mineral oil used as the base oil, magnesium sulfonate and zinc dithiophosphate were added in the quantities listed in Table 1, and, as other additives, an S—P-based package corresponding to GL-4 was added in a quantity of 7.1 wt % to obtain oil samples (b)-(g).

For samples A-C as well as the commercially available oil and oil samples (a)-(g), the properties as well as the wear resistance determined using said wear-resistance evaluation method are listed in Table 1.

From the results of the friction width listed in Table 1, significant effects can be displayed for the oil samples prepared with a low 40° C. dynamic viscosity of 30 mm²/s, corresponding to an excellent effect in increasing the mileage, with the quantity of elemental Mg in the oil at a prescribed value, and with the ratio of the quantity of elemental Zn in the oil to the quantity of elemental Mg in the oil in the prescribed range of 0.2-1. Example 1 was compared with Comparative Examples 2 and 3 with the same quantity of elemental Mg. In Example 1, the ratio of the quantity of elemental Zn in the oil to the quantity of elemental Mg in the oil is 0.67, that is, within the aforementioned prescribed range. On the other hand, in Comparative Examples 2 and 3, the ratio of the quantity of elemental Zn in the oil to the quantity of elemental Mg in the oil is 1.33, that is, outside the aforementioned range, and the wear resistance is much worse.

The lubricant composition for a manual speed-change gear of the present invention with the aforementioned constitution can be used not only as a lubricant for an automobile driving system consisting of a manual transmission (MT), but also for a manual transmission axle (MTX) in transfer, a differential (Dif.), etc. Consequently, it can be used as a common lubricant for said MT, MTX and differential for FF cars, etc. TABLE 1 Invention Example Comparative Example 1 2 3 1-1 1-2⁽²⁾ 2-1 2-2 2-3 2-4 2-5 2-6 Oil Sample A B C ⁽¹⁾ a b c d e f g Quantity of elemental Mg in the oil 0.15 0.30 0.30 0.016⁽³⁾ 0.016⁽³⁾ 0.15 0 0.15 0 0.30 0 (derived from Mg sulfonate) wt % Quantity of elemental Zn in the oil 0.10 0.20 0.10 0.08 0.08 0 0.10 0.20 0 0 0.20 (derived from ZnDTP) wt % Quantity of elemental Zn in the 0.67 0.67 0.33 5.00 5.00 — — 1.33 — — — oil/quantity of elemental Mg in the oil Dynamic viscosity 30 30 30 76 30 30 30 30 30 30 30 Friction width 0.74 0.81 0.80 0.83 1.05 1.00 1.00 0.85 1.03 0.85 0.88 Notes: ⁽¹⁾Commercially available oil for a manual speed-change gear ⁽²⁾Oil sample prepared using a low-viscosity base oil ⁽³⁾Quantity of elemental Ca (derived from Ca sulfonate)

The lubricant composition for a manual speed-change gear of the present invention contributes to protection of the environment since it is an environmentally friendly lubricant by realizing low viscosity. Also, it can be used as a high-quality lubricant for an automobile driving system, such as a manual transmission, manual transmission axle, etc. Consequently, it greatly contributes to the petroleum and automobile industries with regard to manufacture and application. 

1. A lubricant composition for a manual speed-change gear comprising: a base oil; additives comprising at least one alkaline earth metal salt and a zinc dithiophosphate, the amount of alkaline earth metal element based on the total weight of lubricant composition being 0.1 wt % or more, the elemental ratio of zinc to alkaline earth metal from the additive being in the range of 0.2 to 1.0; and wherein the dynamic viscosity at 40° C. of the lubricant is 40 mm²/s or less.
 2. The lubricant composition of claim 1 wherein the amount of alkaline earth metal is from about 0.15 wt % to about 0.6 wt %.
 3. The lubricant composition of claim 1 wherein the dynamic viscosity at 40° C. is 30 mm²/s or less.
 4. The lubricant composition of claim 1 wherein the alkaline earth metal salt is selected from the group consisting of sulfonates, salicylates and phenolates.
 5. The lubricant composition of claim 1 wherein the alkaline earth metal salt is a metal sulfonate.
 6. The lubricant composition of claim 4 wherein the metal sulfonate is magnesium sulfonate.
 7. The lubricant composition of claims 1 and 6 wherein the ratio of zinc to alkaline earth metal is 0.3 to 0.8.
 8. In the method of increasing the wear resistance of manual speed-change gears by lubricating the gears, the improvement comprising: lubricating the gears with a lubricant composition having a dynamic viscosity at 40° C. of 40 mm²/s or less and containing an additive comprising an alkaline earth metal salt and a zinc dialkylthiophosphate, the amount of alkaline earth metal element in the lubricant being 0.1 wt % or more based on the total weight of the lubricant; and the elemental ratio of zinc to alkaline earth metal is from 0.2 to 1.0.
 9. The improvement of claim 8 wherein: the alkaline earth metal salt is selected from the group consisting of sulfonates, salicylates and phenolates; the amount of alkaline earth metal is from 0.15 wt % to 0.6 wt %; and the elemental ratio of zinc to alkalinic earth metal is from 0.3 to 0.8.
 10. The improvement of claim 9 wherein the alkaline earth metal salt is magnesium sulfonate. 